This application claims the priority of Japanese patent application Serial No. 11- 310065 filed on Oct. 29, 1999 and Japanese patent application Serial No. 2000- 281588 filed on Sep. 18, 2000.
1. Field of The Invention
The present invention relates to a shadow mask to be used for color Braun tubes (cathode ray tubes) such as color television display, computer display or the like, and more particularly, to an extension type shadow mask which can be suitably used for an aperture-grill type color Braun tube.
2. Related Prior Art
Conventionally, a shadow mask has been used for color Braun tubes such as color television display, computer display or the like. The shadow mask is disposed to a predetermined portion in the color Braun tube and is formed with opening (aperture) portions through which electron beam is radiated to a fluorescent surface (phosphor layer) formed to an inner surface of the Braun tube.
The shadow mask takes various structures as the opening portion such as:
(a) a structure that a number of small and round holes are formed;
(b) a structure that a number of small holes each having a rectangular-shape are formed;
(c) a structure that a number of slits are provided so as to align with each other.
The shadow masks formed with the opening portions having structures specified in (a) and (b) are hereinlater referred to as xe2x80x9cpress-type shadow maskxe2x80x9d for the sake of convenience, which is usually manufactured by a press-forming method. On the other hand, the shadow mask formed with the opening portion having a structure specified in (c) is hereinlater referred to as expansion-type shadow maskxe2x80x9d for the sake of convenience, which is generally called an aperture grill. The expansion-type shadow mask is usually manufactured by fixing a mask body to a rigid steel frame while the slits are expanded in a longitudinal direction thereof so as not to cause a disturbance in slit interval, i.e. so as to have a uniform interval therebetween.
In the Braun tube provided with such shadow mask, a part of electron beams radiated from an electron gun collides with a surface of the shadow mask without passing through the small holes or the slits. Therefore, the shadow mask is heated by the collision of the electron beams.
In this regard, since the conventional shadow mask is formed of a low-carbon steel plate having a large thermal expansion coefficient, the shadow mask is liable to cause thermal expansion due to the heat generation based on the collision of the electron beams, causing phenomena such that the small holes are displaced in positions and deformed, and slacks are arisen at the slit portions thereby to cause the disturbance in slit interval. Such phenomena leads to the displacement of the electron beams reaching the fluorescent surface inside the Braun tube, so that there has been posed a problem of causing a color mismatching to an image reproduced on the Braun tube.
Further, in the prior art, on the assumption that a metal material (element) to be formed into the expansion-type shadow mask will be thermally expanded, an expanding/fixing working has been performed to the conventional shadow mask by applying a high tensile force to the shadow mask so as not to cause a slack at the slit portions even if the thermal expansion be caused. In a case where the fixing working is performed by applying such high tensile force, the shadow mask is required to be formed from a material having strength capable of enduring the high tensile force. Furthermore, it is also required for the steel frame to be increased in its structural strength.
To cope with these problems, from the viewpoint of suppressing the thermal expansion of the metal material, it has been reviewed and tried to use a metal material having a small thermal expansion coefficient. For example, some of the shadow masks have been formed by using iron-nickel alloy or iron-nickel-cobalt alloy. Such shadow masks have an advantage of that the thermal expansion hardly occurs even if the heat generation is took place by the collision of the electron beams.
However, inherently, the iron-nickel alloy and the iron-nickel-cobalt alloy have not a sufficient mechanical strength. Therefore, when such the metal materials are specifically applied to the expansion-type shadow mask, there may be caused a fear that the materials cannot endure the expanding/fixing working performed under using the high tensile force.
Furthermore, the expansion-type shadow mask is formed from a thin metal material having a small thickness in comparison with the press-type shadow mask which is manufactured through a press-forming so as to maintain the shape coincident with a curved surface of the Braun tube. Therefore, the problems described hereinbefore had become more remarkable.
An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a shadow mask, specifically, an expansion-type shadow mask, capable of being suitably applied to an aperture-grill type color Braun tube, suppressing thermal expansion and sufficiently enduring the expanding/fixing working to be performed under a high tensile force being applied.
This and other object can be achieved according to the present invention by providing a shadow mask, specifically, an expansion-type shadow mask, composed of an iron-based alloy containing 31.0-38.0 weight % of nickel and 1.0-6.5 weight % of cobalt, wherein the iron-based alloy contains 0.01-0.10 weight % of carbon.
According to the present invention of the characters mentioned above, since the iron-based alloy containing 31.0-38.0 weight % of nickel and 1.0-6.5 weight % of cobalt has a small thermal expansion coefficient, the expansion-type shadow mask manufactured by using such a metal material hardly cause a thermal expansion even if the heat is generated through the collision of the electron beam. As a result, the thus manufactured expansion-type shadow mask hardly cause a slack at the slit portions due to the thermal expansion and hardly cause any disturbance of slit-interval, so that the shadow mask is no need to be expanded and fixed by being applied with an excessively high tensile force at the assembling process for the shadow mask, and also, the strength of the steel frame is no need to be increased.
In addition, the iron-based alloy contains 0.01-0.10 weight % of carbon, so that a tensile strength of the alloy can be increased in comparison with an iron-based alloy containing no carbon. Therefore, on the assumption that a certain amount of thermal expansion is caused in the manufacturing process of the expansion type shadow mask, and even if the thermal expansion is actually caused slightly, it becomes possible to perform the expanding and fixing working by applying a high tensile force of a level without causing any slacks at the slit portions.
As a result, the expansion-type shadow mask of the present invention would not pose a fear of causing disturbance of the slits provided in a Braun tube and the displacement of the electron beam on the fluorescent surface formed in the Braun tube would not occur, so that a color-mismatching is not caused in a reproduced image on the Braun tube.
In another aspect of the present invention, it is preferable that the iron-based alloy used in the above expansion-type shadow mask has a tensile strength of 650 N/mm2 or more and 1000 N/mm2 or less.
According to the invention of this aspect, the iron-based alloy has a tensile strength of 650 N/mm2 or more and 1000 N/mm2 or less, so that the expanding/fixing operation for the shadow mask can be easily and appropriately performed by applying a high tensile force at the manufacturing process of the expansion-type shadow mask. As a result, even if the thermal expansion slightly occurs due to the heat generation caused by the collision of the electron beam, the slacks would not occur at the slit portions of the thus obtained expansion-type shadow mask, so that the disturbance of the slit-interval can be effectively prevented.
More in detail, the above-mentioned object of the present invention can be achieved by providing a shadow mask particularly of an expansion type comprising:
a frame member;
a mask body formed in a plate shape as a grid element and mounted to the frame member; and
a plurality of slits formed to the mask body,
wherein the mask body is composed of an iron-based alloy containing 31.0-38.0 weight % of nickel and 1.0-6.5 weight % of cobalt, the iron-based alloy containing 0.01-0.10 weight % of carbon.